1. Field of the Invention
This invention relates to a liquid crystal image projection system for modulating a light beam by a optical image formed in a liquid crystal panel, and projecting it on a screen by means of a projection lens.
2. Description of the Prior Art
In order to obtain a television picture on a large screen, a method of passing a light beam emitted from a light source through a small-sized light valve having formed therein an optical image corresponding to a video signal, and projecting an enlargement of this optical image onto the screen is known. Recently, the method of using liquid crystal as the light valve has been proposed. For example, Morozumi et al. proposed, in "LCD Full-Color Video Projector," SID 86 Digest, p. 375, a projection type display device using an active matrix type liquid crystal light valve. An example of the basic conventional structure of such liquid crystal light valve optical device is shown in FIG. 11.
The light emitted from a lamp 1 is converted into nearly parallel light by a light collecting device 2, and enters a projector lens 4 through a liquid crystal panel 3. In the liquid crystal panel, an optical image is formed corresponding to the video signal as a change in the transmittance, and this optical image is magnified and projected on a screen 5 through the projector lens 4. The liquid crystal panel 3 is formed by joining two spaced opposed glass substrates 6, 7 by applying a sealant 8 on the periphery, filling the internal enclosed space with a liquid crystal material 9 in twisted nematic (TN) mode, and disposing polarizers 10, 11 outside the glass substrates 6, 7. By applying an electric field to the liquid crystal layer of the liquid crystal panel 3, the transmittance of the liquid crystal panel 3 can be controlled.
As one of the methods for driving the liquid crystal panel, the active matrix system is known, in which switching elements such as thin film transistors (TFTs) and nonlinear elements are connected to the pixel electrodes. FIG. 12 shows an equivalent circuit of the active matrix liquid crystal panel using TFTs. Near the intersections of scanning electrodes X.sub.1, X.sub.2, . . . , Xn and signal electrodes Y.sub.1, Y.sub.2, . . . , Ym formed in a matrix, TFTs 12 are formed as switching elements, and the gate and source of each TFT are connected to the corresponding scanning electrode and signal electrode. Each pixel electrode is connected to the drain of a corresponding TFT. The liquid crystal layer of one pixel may be regarded as a capacitor.
The scanning electrodes and signal electrodes are connected to a scanning circuit 13 and a signal supply circuit 14, respectively. The scanning circuit 13 is of linear sequential type, and sequentially scans the scanning electrodes X.sub.1, X.sub.2, . . . , Xn, to simultaneously turn on all TFTs connected to each scanning electrode and turn off all other TFTs. Synchronizing with this scanning, signals are applied to the pixel electrodes connected to the TFTs in ON state through the signal electrodes Y.sub.1, Y.sub.2, . . . , Ym from the signal supply circuit 14. While the TFTs are OFF, the pixel electrodes are maintained at respective constant voltages.
Thus, in the active matrix system, using multiple scanning electrodes, images of high quality can be formed, so that the liquid crystal image projection system is considered ideal for obtaining television images on a large screen.
However, the active matrix system involves the following problems. When a direct current is continuously applied to the liquid crystal material, electric and optical characteristics deteriorate. Thus, generally the pixels are driven by an AC voltage so that DC components will not be applied to the liquid crystal materials. The simplest method is to invert the polarity of the signal voltage in every scanning period.
In the active matrix system, since the OFF resistance of the switching elements is not perfectly infinite, when the polarity of the signal voltage is inverted in every specific period, the absolute value of the voltage to be held by each pixel is lowered with the passing of the time, and the effective voltage applied to each pixel becomes smaller than the absolute value of the signal voltage. By entering parallel light into the liquid crystal panel and applying identical signal voltages to the pixels, the brightness of the projected image varies along the scanning direction. This is called the brightness gradient
The brightness gradient may be improved to a certain extent by modulating the signal voltage in the scanning period. But, since the service range of the signal voltage-transmittance characteristics becomes narrow, gradation characteristics will not be sufficient. Further, if the brightness gradient is too large, correction is impossible.